Found: a “fluffy” galaxy made from 99% dark matter

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Dark matter is weird and frustrating. We’ve never seen it, since one of its properties is that it doesn’t interact with normal matter or with electromagnetic radiation at either end of the spectrum. Current estimates suggest that dark matter is far more common than ordinary matter, constituting between three-quarters and 84.5% of the total mass of the universe. In our own galaxy, dark matter is thought to be about five times more common than ordinary matter – but now scientists working with the Gemini North and Keck II telescopes have discovered a galaxy that’s made from 99.99% dark matter. And they noticed it because it was “fluffy.”

This new galaxy, dubbed Dragonfly 44, isn’t the first dark matter galaxy we’ve found, but the others have been comparatively tiny. Dragonfly 44, in contrast, is roughly the size of the Milky Way, but only 1% as bright: like a “wisp of cloud” compared to our home galaxy. Sure, Dragonfly 44 has stars, but not very many of them. And they’re in a loose, fluffy distribution. So why are the stars still hanging out so close together, when they’re in a “dense, violent region of space” so likely to shake them down for spare atoms?

In order for the galaxy to hold together at all, dark matter has to be gluing it together. Nothing else explains the speed of the stars that make up Dragonfly 44, in the face of its relatively small observable mass. In fact, even in the central regions of the ultra-diffuse galaxy, where the few remaining visible stars are concentrated, dark matter accounts for an estimated 98% of the mass. This is an unusual finding in an otherwise normal galaxy. As study author Pieter van Dokkum told the Washington Post:

“If it’s a very big or very large galaxy, you can brush it off and say, oh, that must be a rare thing,” he said, “but most of the stars in the universe live in galaxies this size.”

“We thought that that ratio of matter to dark matter was something we understood. We thought the formation of stars was kind of related to how much dark matter there is, and Dragonfly 44 kind of turns that idea on its head,” he continued. “It means we don’t understand, kind of fundamentally, how galaxy formation works.”

The dark galaxy Dragonfly 44. The image on the left is a wide view of the galaxy taken with the Gemini North telescope using the Gemini Multi-Object Spectrograph (GMOS). The close-up on the right is from the same very deep image, revealing the large, elongated galaxy, and halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way Galaxy. Dragonfly 44 is very faint for its mass, and consists almost entirely of dark matter. Credit: Pieter van Dokkum, Roberto Abraham, Gemini, Sloan Digital Sky Survey

The reason we infer the existence of dark matter in the first place, despite still not being able to observe it directly, is that our current understanding of physics demands the existence of something. Here’s the problem in a nutshell: There’s far too much energy in the Milky Way and other galaxies to allow them to retain their present configurations if dark matter doesn’t exist. The mass of the visible objects within the galaxy (stars, planets, nebulas, etc) isn’t nearly high enough to explain the speed at which the galaxy rotates. The rotational velocity of the Milky Way and other galaxies is so high that they’d fly apart if the only thing holding them together was the mass of the observable universe. Except, of course, they don’t fly apart – which means something else is contributing mass and holding things together. Decades of analysis has demonstrated that dark matter exists – we see evidence of it in gravitational lensing (the way light bends in the presence of a strong gravitational field), and in measurements of cosmic background radiation.

The team is hoping to find new examples of dark matter galaxies closer to home. One of the predictions about dark matter is that when particles of dark matter (weakly interacting massive particles, or WIMPs) interact with each other, tiny flashes of ultraviolet light are produced. In a typical galaxy, these are drowned out by the light of stars and stellar events. In a true dark matter galaxy, however, things might just be dark enough to make them out – at least if the galaxy is sufficiently close to Earth.

Dragonfly 44 is named for Dragonfly, an array of sensitive telephoto lenses that van Dokkum and several other colleagues assembled for use as an unconventional if effective telescope. The area of space where the galaxy lies is known as the Coma galaxy cluster and it’s an estimated 330 million light years away. 47 similarly faint galaxies were observed, far dimmer than the other galaxies within the cluster.

How these galaxies form is still a mystery. Nature reports that a quasar at the heart of the galaxy may have destroyed the gas reserves that normally would’ve formed conventional stars, or that interactions with other galaxies in the Coma cluster might be responsible for the phenomena. Either way, it’s a highly unusual find that might one day help us understand more about the material that constitutes much of the universe.

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